let promise = new Promise(function(resolve,reject){
    setTimeout(function(){
        let a = 'hello ';
        resolve(a);
    },10);
});
promise.then(function(a){
    return new Promise(function(resolve,reject){
        setTimeout(function(){
            let b = 'lagou ';
            resolve(a+b);
        },10);
    });
}).then(function(b){
    return new Promise(function(resolve,reject){
        setTimeout(function(){
            let c = 'I love U';
            resolve(b+c);
        },10);
    });
}).then(function(result){
    console.log(result);
});